Support #empty_option

We have been using the following to add the empty option to the zendesk options field. 
```
'#zendesk_extra_parameters' => array(
  '#additional_options' => array(
    '' => t('Select your subject'),
  ),
),
```
We should support the attribute #empty_option like Drupal does. 
```
'#empty_option' => t('Select your subject'),
```